1. Field of the Invention
This invention relates to zoom lenses, and more particularly to high range zoom lenses of reduced total length suited to still cameras, cine cameras or video cameras.
2. Description of the Prior Art
The cameras for photography and video cameras have demanded for zoom lenses of increased magnification ratio while still maintaining the total length of the lens at a minimum. To achieve an increase in the magnification ratio of the zoom lens, it has been customary to increase the total movement of the lens unit for varying the magnification, or to heighten the refractive power of the lens unit for varying the magnification, or to increase the number of lens units movable for zooming. However, these methods have been accompanied by drawbacks of increasing the total length of the system, raising variation of aberrations with zooming, and increasing the complexity of structure of the operating mechanism.
For example, Japanese Laid-Open Patent Application No. SHO 54-30855 discloses a zoom lens comprising three or four lens units, three of which are made movable for zooming to achieve an increase of the zoom ratio. Because of the increased number of movable lens units, the complexity of structure of the operating mechanism tended to increase.
Meanwhile, recently, a novel focusing method called "rear-focus" or "inner-focus" has been proposed. This method has advantages that the diameter of the outer lens barrel can be reduced, the weight of the focusing lens unit is so light as to accommodate the auto-focus capability, and the focusing speed is improved. However, in application of the "rear-focus" or "inner-focus" method to zoom lenses, there has been a problem that the focusing movement for the same object varies with variation of the focal length.
To lighten the above-described problem, according to the prior art in Japanese Laid-Open Patent Application No. SHO 58-91421, the image magnification of the lens unit movable for focusing is made variable. This method has been expanded in Japanese Laid-Open Patent Application No. SHO 58-136012 where two of the lens units which contribute to variation of the image magnification are made to move when focusing so that the change of the focusing movement resulting from the change of the focal length is controlled. Other previous proposals are for moving two lens units at different speeds from each other when focusing as in Japanese Laid-Open Patent Application Nos. SHO 58-98720, 58-211117, 58-211118, and for moving two or three lens units which when zooming move in differential relation, as a unit when focusing, as desclosed in Japanese Laid-Open Patent Application Nos. SHO 60-39613 and 60-14212.
The above-described proposals each have amied at how the focusing movement is less affected by the zooming position. In actual practice, however, as consideration must be taken up to the lens mounting mechanism, its operation and driving, the situation is complicated, the weight of the focusing lens unit or units is heavy, and the size of the focusing unit including an operating mechanism for controlling the complicated focusing movement becomes very large. In Japanese Laid-Open Patent Application No. SHO 60-43619, there is proposed a focusing mechanism for three lens units of complicated movement.